What if the idea is that he wants to have sex and would have it if he could, but knows that in all likely hood he probably won't be able to get that opportunity.

This differs from asexual, which means he doesn't want it in the first place.
---------- That said, "sexual atheist" could possibly mean that he rejects the idea that sex as a whole exists at all. This is a matter to be debated though, and either way is not the intent of the picture at all.

It's been discussed a good bit, but according to www.asexuality.org/home/
"An asexual is someone who does not experience sexual attraction."
They also state that what it means differ from person to person.

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Actually, asexuality means that you don't feel the need for having sex or have from very small amount to none pleasure from having sex.
Asexuality isn't a sexual orientation (= hetero-, homo-, bi-, and pansexuality) though.